FIG. 1 is a front view illustrating the installation structure of a column and a girder or a slab according to the related art.
The installation structure of FIG. 1 includes columns 10 set up at regular intervals, girders or slabs 20 connected between the adjacent columns 10.
The girder or slab 20 is directly connected to the center or side of the column 10, and the connected girder or slab sags under the weight or load of an installation (not shown) placed thereon.
According to a standard diagram handbook of machine design, uniformly distributed load can be calculated by the following formula.δmax=5wL4/384EI 
where δmax: a quantity of maximum sagging
w: load
L: length
E: Young's Modulus
I: secondary moment of area
The quantity of maximum sag (δmax) is proportional to the fourth square of the whole length L of the girder or slab.
In FIG. 1, the whole length L corresponds to an effective length l of the girder or slab 20 between the columns 10 where sagging occurs, and the maximum sagging (δmax) corresponds to bent displacement e that is the length of sagging at the center of the girder or slab 20.
However, in such installation structures, the effective length l of the girder or slab 20 is too long, so that the sag occurring on the girder or slab 20. To avoid this problem, a girder or slab 20, the secondary moment I of area of which is high, should be used Thus, a girder or slab 20 with larger thickness and size is required, which problematically increases the cost of the girder or slab greatly.